The present invention relates to an electrophotographic element and, more particularly, to an electrophotographic photoconductor that includes a novel intermediate layer and exhibits superior and stable image quality characteristics.
Until recently, an electrophotographic photoconductor element (hereinafter also referred to as a "photoconductor") used in conjunction with the electrophotographic device invented by Carlson generally utilized inorganic photoconductive materials such as selenium, a selenium alloy, zinc oxide, or cadmium sulfide. In recent years, however, many photosensitive elements using organic photoconductive materials have been developed with the aim of taking advantage of their non-noxiousness, good film forming capability, light weight, and low cost.
Of particular interest has been the development of laminated organic photoconductors with a photosensitive layer divided into function-specific layers (hereinafter also referred to as "function-separated, laminated photoconductors"), namely a charge-generation layer that receives light to generate charge carriers, and a charge-transfer layer that transfers generated charge carriers. Many such photoconductors have been developed and used in conjunction with electrophotographic devices such as copying machines, printers, and facsimile machines because such photoconductors offer many advantages. For example, individual function layers can be separately formed of the materials best suited for the desired functions and later combined, thereby substantially increasing the device sensitivity. In addition, spectral sensitivity can be improved depending upon the wavelength of the exposure light.
Most function-separated, laminated organic photoconductors that have been practically applied include a photosensitive layer composed of a charge-transfer layer on top of a charge-generation layer, which in turn is laminated on a conductive substrate. The initial step in manufacturing such a photoconductor is sublimating and depositing an organic charge-generation material on a conductive substrate to form a charge-generation layer. Alternatively, the charge-generation layer may be made by coating, and later drying, the conductive substrate with a coating liquid that is made by dispersing and dissolving an organic charge-generation material and a binder in an organic solvent. Subsequently, a charge-transfer layer is formed by applying, and later drying, a coating liquid that is made by dissolving an organic charge-transfer material and a binder in an organic solvent.
Fundamentally, such a configuration for a photosensitive layer satisfies the basic requirements of a photoconductor for image formation. However, in a practical context, it is important to ensure good images with minimal defects, and good image quality must be maintained over long periods of repeated use. Thus, the photosensitive layer should be a homogeneous, defect-free film having superior electrical properties, and the film quality and the electrical properties should not deteriorate or become unstable after long periods of use.
As is well known to those skilled in the art, it is desirable that the charge carriers generated by the charge-generation layer be able to move fast and be fed into the conductive substrate or the charge-transfer layer instead of being recombined with free electrons and disappearing or being trapped. Thus, the charge-generation layer should preferably be as thin as possible, and currently available photoconductors usually incorporate a charge-generation layer with a thickness in the order of submicrons. However, because the charge-generation layer is formed as such a thin film, contamination, irregularities in shape, and roughness of the surface of the conductive substrate directly result in irregularities in the charge-generation layer. The irregularities in turn cause image defects such as voids, black points, or non-uniform density.
Typically, an aluminum alloy cylinder or a cylinder which has a surface that has been smoothed by cutting and polishing may be used as the conductive substrate. However, the surface roughness of the substrate, contamination of the surface, dispersion of the amount or size of deposits of the metal contained as the alloy component, and surface irregularities caused by the dispersion of the oxidation of the surface result in non-uniform film formation in the charge-generation layer formed on the surface. This result substantially reduces the quality of the images obtained.
In order to avoid such irregularities in the film, and in order to obtain a "blocking effect," which prevents a decrease in the charge-retaining capability of the photoconductor caused by positive holes injected from the conductive substrate when needed, an intermediate layer of an N-type resin with a low electric resistance has been provided on the surface of the conductive substrate as a solution. Resins such as solvent-soluble polyamide, polyvinylalcohol, polyvinylbutyral, or casein have long been used to form the intermediate layer for the above-described reasons. With such resins, even very thin films, for example, films of 0.1 .mu.m or less, can adequately provide a blocking layer effect, provided that no other function is required of the resin.
However, if the resin layer is to serve other functions, e.g., covering the irregular contour and smoothness of the surface of the conductive substrate, and preventing non-uniform distribution of the charge-generation coating liquid to avoid non-uniform film formation, a film thickness of 0.5 .mu.m or more is required. In some cases, a thickness of several tens of .mu.m is required depending upon the machining conditions of the substrate and the contamination of the surface. If a resin layer of such a thickness is formed of polyvinylalcohol, solvent-soluble polyamide, or casein, however, the residual potential is increased and the electrical properties of the photoconductor is subject to change as a function of changes in temperature and humidity. This problem occurs because the resin layer is characterized by high water absorption, and the electrical conductivity of the resin layer is easily changed by the moisture contained in the layer since conductivity mainly depends upon ion conduction, i.e., the movement of H or OH ions resulting from the dissociation of the water molecules in the layer.
Various materials having a low electrical resistance have been proposed for use as the intermediate layer in a photoconductor which is substantially unaffected by changes in the external environment. For example, Japanese KOKAI 2-193152, Japanese KOKAI 3-288157, and Japanese KOKAI 4-31870 disclose the chemical structures of solvent-soluble polyamide resin to be used as the intermediate layer. Japanese KOKOKU 2-59458, Japanese KOKAI 3-150572, and Japanese KOKAI 2-53070 disclose methods for adding an additive to polyamide resin to prevent any change in the electric resistance as a function of a change in environment. In addition, Japanese KOKAI 3-145652, Japanese KOKAI 3-81778, and Japanese KOKAI 2-281262 disclose methods for mixing polyamide resins with other resins to adjust the electrical resistance and to reduce the electrical resistance's susceptibility to change as a function of change in the environment. However, because these methods teach the use of polyamide resin as the principal material, the effects of temperature and humidity levels cannot be completely avoided.
Other previously disclosed methods include using cellulose dielectrics (Japanese KOKAI 2-238459), polyetherurethane (Japanese KOKAI 2-115858 and Japanese KOKAI 2-280170), polyvinylpyrrolidone (Japanese KOKAI 2-105349), or polyglycolether (Japanese KOKAI 2-79859) as the intermediate layer. Alternatively, the use of a cross-linked resin has been proposed to prevent the amount of moisture in the resin layer from being affected by a change in the environment. Furthermore, methods using melamine resin (Japanese KOKAI 4-22966, Japanese KOKOKU 4-31576, and Japanese KOKOKU 4-31577) or phenol resin (Japanese KOKAI 3-48256) are also known. However, effectiveness of such methods are limited by the fact that, when the required resin layer is relatively thick, for example, several .mu.m, the resistance and the residual potential are increased.
One method for counteracting the above-mentioned drawback is to utilize electron-conduction device physics instead of ion-conduction device physics in connection with the material forming the intermediate layer. One of the methods based on this idea is a method that provides a resin layer by dispersing conductive powder such as tin oxide or indium oxide (Japanese KOKOKU 1-51185, Japanese KOKOKU 2-48175, Japanese KOKOKU 2-60177, and Japanese KOKOKU 2-62861). However, if this method is used, it is difficult to make a resin coating liquid having uniformly dispersed conductive powder while stably preserving the coating liquid without having the conductive powder separate or settle. Furthermore, very small protrusions on the surface of the coated resin layer are often caused by the separation and agglomeration of the conductive powder. Such protrusions cause defects in images provided by the photoconductors.
Yet another known method involves using an organic metal compound instead of conductive powder to form a coating liquid. In this method, as disclosed in Japanese KOKOKU 3-4904 and Japanese KOKAI 2-59767, the organic metal compound and resin are dissolved in an organic solvent in order to form an intermediate layer. However, the coating liquid used in this method is unstable, and many additional problems must be solved before this method can be applied practically to mass production.
Given the above problems associated with using a resin layer as the intermediate layer provided on the conductive substrate, it is the object of this invention to provide a photoconductor that has superior electrical properties and superior image quality which are substantially unaffected by environmental factors, while facilitating high productivity.